A digital oscilloscope is used to capture, analyze and display electronic signal waveforms. The digital oscilloscope samples the voltage amplitude of an input signal and performs an analog-to-digital conversion (ADC) of the voltage values of the acquired samples. The digitized information is stored in memory and then used to create the display of the input signal's waveform on a cathode ray tube (CRT) display screen. Conventionally, the oscilloscope display is one of voltage-versus-time where units of voltage amplitude are plotted along the vertical axis and units of time are plotted along the horizontal axis. Typically, a wide selection of both voltage and time scale ranges is available to the user. The conventional voltage-versus-time plot may be regarded as a time-domain display of the input signal waveform. The time-domain display provides the user with critical phase and frequency information for a given input signal waveform.
The time-domain display, however, does not provide a complete display of the waveform. In particular, the relative frequency of occurrence of waveform voltage amplitude levels is not readily discernible in the time-domain. Determining the frequency of occurrence of amplitude levels is critical for several reasons. For instance, in pulse waveform analysis it is necessary to determine the base and top magnitudes, that is, amplitude levels, of the pulse. The base and top magnitudes of a pulse provide amplitude information used to establish the distal, mesial, and proximal lines. These lines typically designate the 90 percent, 50 percent and 10 percent values, respectively, of the pulse amplitude. These values are used extensively in timing analysis. For instance, the risetime of a pulse is defined as the amount of time required for the pulse to rise from 10 percent to 90 percent of its final amplitude value.
A standardized technique for determining the base and top magnitudes of a pulse waveform requires the computation and analysis of a probability-density histogram of the pulse data. (This standardized technique is described in the Institute of Electrical and Electronic Engineers (IEEE) Standard 181-1977, Section 4.3, entitled Pulse Measurement and Analysis by Objective Techniques.) The probability-density histogram is essentially an analytical bar-chart giving the frequency of occurrence of amplitude levels. The probability-density histogram may be regarded as a probability-domain analysis of a waveform in contrast to the conventional voltage-versus-time time-domain display of an oscilloscope. Conventionally, the probability-domain analysis, if performed, is invisible to the user of the oscilloscope.